The proposed research explores functional properties of the nasal and ocular common chemical sense (CCS) (i.e., nasal and ocular trigeminal chemoreception) and of the sense of smell. In order to separate the trigeminal from the olfactory response of the nose, ocular and nasal detection thresholds to airborne chemicals will be measured in persons lacking olfaction (i.e., anosmics) and in age-, gender- and smoking- status-matched controls (i.e., normosmics). Anosmics can provide odor unbiased nasal pungency thresholds (i.e., sensations like tingling, piquancy, burning, irritation, prickling, freshness, stinging, and the like) while normosmics will provide odor thresholds. Trigeminal chemical sensitivity of the conjunctiva (eye irritation) will also be measured. Stimuli will comprise homologous series of compounds (e.g., aldehydes, carboxylic acids, terpenes). Physicochemical properties change systematically in such series. Thus, it will be possible to track down how these changes reflect themselves in the stimulus ability to evoke thresholds for nasal pungency, eye irritation, and odor, allowing us to probe the physicochemical determinants of the three sensations. Once information is gathered on a few dozen individual substances, we will measure thresholds for the same three sensory responses using mixtures of chemicals varying in number and type of components. The results will provide information on the extent and basis for sensory additivity of nasal pungency, eye irritation and odor at threshold level, and will highlight salient features of the reception process(es) at work. We will compare, in anosmics, the quality discrimination ability of the nasal and ocular trigeminal systems when stimulated with different series of homologous compounds presented at equal-intensity levels. We will also explore, both for anosmics and normosmics, thresholds for localization of stimuli to the right or left nostril. The outcome will serve as a basis for a classification of trigeminal stimuli. It will also allow us to relate physicochemical commonalities among substances in the same trigeminal perceptual category to a shared reception process.